Ultra low power analog circuits often require a current reference. Known self biased current generators may be used for this purpose, however, starting up such a current source is not straightforward which presents a problem.
Starting analog circuits has always been tricky. For example, making sure a same exact state is reliably reached after power is turned-on repeatedly has been a challenging engineering problem. Current approaches suffer weakness such as being affected by process variations, not starting up repeatedly if the power cycles are too close in time, etc. This presents a problem.
Additionally most approaches address high current applications. Ultra low power circuits need special attention to current consumption, so solutions for high current applications present a problem for ultra low current circuits.
FIG. 3 illustrates a prior approach, generally, at 300. At 302 is voltage rail such as Vdd, and at 304 a ground rail. FIG. 3 illustrates a simplified circuit for a self-biased current source. This current source may have two stable points one of them which gives zero (0) current. This presents a problem.
FIG. 4 illustrates a prior approach, generally, at 400. At 402 is voltage rail such as Vdd, at 404 a ground rail, at 406 a capacitor C, and at 408 a node. FIG. 4 illustrates one attempt at avoiding the zero current startup mode. A capacitive discharge technique is used. In this implementation, node 408 is pulled to Vdd when the power supply (such as Vdd) ramp is applied. A problem with this approach is that it is not guaranteed to have C discharged every time the power ramp is applied. Also depending on the charge that C carries during the power ramp, the circuit may or may not reach the stable non-zero current operating point. It is possible that the circuit may revert be back to “zero” current mode. Besides, any transient may affect the situation in such a way that a circuit may start before supply voltage reaches to a usable value. This may be prone to have transient effects more. Start up time is not really guaranteed. This presents a problem.